Lapping machines and other manual tools for machining surfaces are commercially available that use a working disc screw-fastened to a hub eccentrically and rotatably held by a support casing made to rotate on its own axis by a suitably-operated driving shaft.
In these tools the problems arises of assemblying or removing the working disc when its replacement is necessary. Such operations require that the rotation of the rotating hub be stopped while the disk is screwed or unscrewed.
The greatest difficulties do not arise so much in the screwing or unscrewing operation of the working disc, that can often be done by hand, but in preventing the rotation of the hub, that generally requires the use of a spanner or other tool, with the consequent design of the tool casing with holes, operating spaces and so on.
Problems are even more evident in those tools equipped with a dust collection system, where suitable shrouds are provided that make for difficult access to the rotating hub from the outside.
One known solution that allows the rotation of the hub to be stopped without any direct action on the hub provides for the hub to have two diametrically-opposed flat milled portions on the external surface of the hub, and for the working disc to have an upper tubular end with a diameter such that it can be engaged on the hub, which end is also provided with two diametrically-opposed sections with a flat surface; when the disc is engaged on the hub, the two pairs of flat surfaces prevent relative motion between disc and hub, so that, if the disc is held with one hand, the rotation of the hub is prevented, and it becomes possible to screw up the working disc on the hub with an Allen screw inserted in a through hole in the centre of the disc.
However, the solution described has the drawback that it requires an Allen screw, not always immediately available, and there is also the risk that the Allen screw is lost.